The invention relates to a method for the manufacture of a molding, in particular through selective laser melting, according to the pre-characterizing clause of Claim 1, and to a sensor unit, in particular for application of the method.
The present invention refers to a technology known as “rapid prototyping”. Such methods are preferrably used in product development, in order to reduce the product development time and to increase the product quality. In addition, this method allows manufacture of complex geometric shapes directly from a three-dimensional CAD model. For example, such a method has been disclosed in DE 196 49 865 C1.
The material powder is distributed over the build platform or the build space by means of a levelling assembly, for example through a linear movement of a slide levelling the material powder to the layer thickness desired. Whenever a layer has been treated with the laser beam, the levelling assembly distributes new powder over the component part that has been lowered by the thickness of the next layer to be applied. To allow a uniform application, EP 1 234 625 A1 discloses a levelling assembly which permits uniform application of the next powder layer to be treated, irrespective of any surface irregularities and elevations of the previously treated layer. To achieve this, said levelling assembly comprises at least a number of individual elements which can be displaced when moving over elevations. Preferred use is made of brushes or brushes arranged in a line.
In essence, the quality of the component parts depends on maintaining an exact thickness of the first powder layer which is applied onto a build platform or a base plate mounted on the build platform. If the first powder layer applied is too thick, there is the risk that the layer fails to comprise a sufficiently secure connection to the base plate. If the powder layer applied is too thin, this might result in height errors which may be continued while further layers are being built. If the levelling assembly is a rigid one, such as a slide or a grinding shaft, there is the risk that the levelling assembly collides with the build platform or the base plate if the powder layer is too thin. The presently manual adjustment of the build platform for achieving an exact thickness of the first powder layer is difficult in that the build platform is preheated to temperatures in excess of 250 degrees C., in order to prevent internal stresses from developing while the molding is being built. Where generative methods such as selective laser melting are utilized, the build platform is heated to temperatures of up to 500 degrees C; in exceptional cases, there might even be higher temperatures of up to 600 degrees C. This heating process causes considerable thermal expansions which also affect the position of the build platform. This complicates the adjustment of an exact thickness of the first powder layer for the manufacture of a molding.